This girl I hate knows that I know that she pissed herself?

Simple. Next time you see her tell her to back off from your friends or you'll let her little secret slip and really make her look silly. Her choice but it's a good hold to have over her. Mind you. it depends on her personality. If I'd done that I'd admit it, make a joke of it and turn it around by saying 'Well, what would you have done in my situation?' so be careful to ess whether she'd do that first.